Detailed investigations were performed to characterize lamprey fibrinogen, the most primitive of the vertebrates, as to its kinetic-molecular properties and mode of action. Highly purified preparations via sedimentation-equilibrium gave a molecular weight of 365,000. The individual chains were also examined. The protein was also studied by differential scanning calorimetry. The results of clotting this protein by enzymes other than thrombin were examined by electron microscopy.